The present disclosure relates to a memory or logic device based on a magnetic domain wall (MDW), and in particular, to a technology of controlling a position of a MDW in a MDW-based memory or logic device.
As the information industry develops, it is necessary to process a large amount of information, and thus, there is a continuously increasing demand for a memory or logic device, which is configured to store or process such a large amount of information. A hard disk drive (HDD) is widely used as a storage medium, to or from which information is written or read. The HDD is generally used to store a large amount of information (e.g., larger than several TeraBytes), and the reading and writing operations of the HDD may include rotating a mechanical head and a disk using a rotating component. However, the presence of such a mechanically rotating component means that the HDD suffers from mechanical abrasion or operational failure, which may lead to low reliability in data storing characteristics of the HDD. In addition, due to the presence of the mechanical component, the HDD is vulnerable in terms of stability and portability, and thus, there are several difficulties in using the HDD as a next-generation information storage medium.
A memory device, in which a ferromagnetic material with magnetic domain wall is used, has been proposed as a next-generation information storage medium, and there are many studies on such a ferromagnetic memory device. A ferromagnetic material may include a region, which has the same magnetization direction and is called ‘a magnetic domain’. When the ferromagnetic material has several magnetic domains, adjacent ones of the magnetic domains may have different magnetization directions and an interface therebetween is called a magnetic domain wall. Magnetic domains in a ferromagnetic material have sizes and magnetization directions that can be changed by a magnetic field or an electric current applied from the outside, and this property of the ferromagnetic material may be used to move the magnetic domain wall therein.
Stuart Parkin, a physicist at IBM's Almaden Research Center, in San Jose, Calif., proposed a magnetic racetrack memory using the motion of a magnetic domain wall caused by an electric current, not using the motion of the mechanical component. In such a magnetic racetrack memory, opposite magnetization directions of magnetic domains are stored as data in a magnetic nanowire. A data writing operation of the magnetic racetrack memory may include a step of changing a magnetization direction of a magnetic domain, and a magnetic field produced from a magnetic domain wall of a writing device may be used for the data writing operation. A data reading operation of the magnetic racetrack memory may include a step of reading a difference in resistance of a magnetic tunnel junction of a reading device attached to the magnetic nanowire. If an electric current is applied to flow through a magnetic nanowire, magnetic domains and magnetic domain walls of the magnetic nanowire may be moved at the same time, and this makes it possible to access data stored in a desired position of the magnetic nanowire.
While an idea and concept of the magnetic racetrack memory have been established, there are many technical challenges that should be overcome to realize the magnetic racetrack memory. For example, in order to move a magnetic domain wall of a magnetic nanowire, it is necessary to increase a current density to be applied to the magnetic nanowire, but this may lead to various problems e.g., heating of the nanowire), and thus, it is necessary to reduce a current density of a current. Also, the motion speed of the magnetic domain wall is slow, and it is necessary to uniformly move many magnetic domain walls by a desired distance and to pin the magnetic domain walls to desired positions. For these purposes, further research is required in the future.